Accessing parity-forbidden d-d transitions for photocatalytic CO2 reduction driven by infrared light

A general approach to promote IR light-driven CO2 reduction within ultrathin Cu-based hydrotalcite-like hydroxy salts is presented. Associated band structures and optical properties of the Cu-based materials are first predicted by theory. Subsequently, Cu4(SO4)(OH)6 nanosheets were synthesized and are found to undergo cascaded electron transfer processes based on d-d orbital transitions under infrared light irradiation. The obtained samples exhibit excellent activity for IR light-driven CO2 reduction, with a production rate of 21.95 and 4.11 μmol g−1 h−1 for CO and CH4, respectively, surpassing most reported catalysts under the same reaction conditions. X-ray absorption spectroscopy and in situ Fourier-transform infrared spectroscopy are used to track the evolution of the catalytic sites and intermediates to understand the photocatalytic mechanism. Similar ultrathin catalysts are also investigated to explore the generality of the proposed electron transfer approach. Our findings illustrate that abundant transition metal complexes hold great promise for IR light-responsive photocatalysis.

In the pursuit of global carbon neutrality, artificial photocatalysis offers a powerful solution to simultaneously address the issues of the greenhouse effect and energy shortage. This process involves the direct transformation of carbon dioxide (CO 2 ) and water into useful fuels and oxygen under ambient conditions 1 . The proper photocatalysts with well-matched band edge positions are the prerequisite requirement to achieve this objective. In principle, the conduction band minimum (CBM) of the catalysts should be more negative than the potential for CO 2 reduction, while the valance band maximum (VBM) needs to be more positive than the potential for water oxidation. For this purpose, the band gaps of the photocatalysts should ideally fall within the range of 1.8-2.0 eV 2 . Recently, the infrared (IR) light-driven redox reactions have gained widespread attention among researchers. Despite the low energy of IR light and its tendency to generate localized heat, its relatively high proportion in the solar spectrum (ca. 50%) has prompted people to explore ways to utilize it. In principle, IR light is theoretically precluded for photocatalytic CO 2 reduction owing to its low photonic energy of less than 1.55 eV 3 . Fortunately, band structure engineering provides a viable strategy to finetune the energy band distribution and optimize the transfer behavior of activated electrons 4 . In this scenario, the step-by-step electron transition mode has proven to be the most attractive strategy to effectively utilize the low-energy photons, which could trigger the concurrent transformation of CO 2 and water into hydrocarbons and oxygen under IR light irradiation 5,6 .
Recently, some ultrathin metallic catalysts (such as CuS and CoS 2 nanosheets) have been reported to offer an activity for IR light-driven CO 2 photoreduction, in which the intraband and interband electron transitions are coupled to realize the absorption of IR light while enabling the redox potential requirements of electron-hole pairs ( Fig. 1a) 7,8 . Besides, some bismuth-based catalysts, like metallic Bi 19 Br 3 S 27 nanowires 9 , has also been reported to exhibit excellent activity for CO 2 reduction under near-infrared (NIR) light irradiation. However, metallic conductors usually suffer from severe recombination of the photoexcited charge carriers, resulting in low efficiency for CO 2 photoreduction. Meanwhile, the serious electron scattering effect and localized plasmon effect of the metallic conductor can give rise to an energy loss of the active electrons and localized lattice heating, leading to the deactivation of the catalysts 10,11 . On the other hand, the defect-induced semiconductors with intermediate bands (e.g., WO 3 nanosheets with oxygen vacancies) also exhibit certain performance for IR light-driven CO 2 reduction, in which the reasonably regulated intermediate band could serve as a step for the cascaded electron transfer processes under IR light excitation (Fig. 1b) 3 . Nevertheless, the limited materials selection and harsh processes of defect regulation retard their further development. As such, there is an urgent need to investigate photocatalytic systems that follow a universal principle for the transition of charge carriers under IR light. Such systems would expand the range of photocatalysis to the IR spectrum and enable the preparation of full-spectrum-response catalysts by combining infrared photoactive catalysts with conventional catalysts.
Transition metal ion complexes are diverse and can be easily prepared, usually exhibiting a high IR light absorption capacity due to the d-d interband transitions 12,13 . In general, the transitions of electrons in d orbitals are parity-forbidden because of the same angular momentum quantum number. However, in some transition metal complexes with tetrahedral or octahedral coordination, like hydrotalcite-like hydroxy salts, the strong p-d orbital coupling between the coordinating groups and the metal ion can result in the degeneracy of valence band d orbitals of transition metals 14 . Therefore, some empty d orbitals appear in the middle of the band gap, which could act as a "cushion step" to make the d-d transition possible and thus induce a strong IR light absorption (Fig. 1c). Nevertheless, like most bulk materials, the excited carriers under IR light irradiation recombine rapidly and cannot be utilized for the catalytic reaction. On account that two-dimension (2D) materials possess ultrathin thickness in one dimension, they could have great potential for electron-hole separation benefiting from the shorter distance for carrier transfer [15][16][17] . To this end, the hydrotalcite-like hydroxy salts with 2D nanosheet structure could be a good candidate for IR light absorption and charge carrier separation so that the activated electrons participate in the photocatalytic reactions.
In this work, we first predict the existence of splitting d orbitals in 2D hydrotalcite-like hydroxy salts and their potential IR responseability by the simulated density of states (DOS), band structures, and optical absorption spectra. Then, ultrathin basic copper sulfate (Cu 4 (SO 4 )(OH) 6 ) nanosheets (CSON) were prepared as the prototype, and an IR light-driven CO 2 reduction reaction was taken as an example to verify the possibility of d-d transition-induced photocatalytic reactions. The basic OH groups on the surface of Cu 4 (SO 4 )(OH) 6 nanosheets provide abundant sites for initial CO 2 adsorption. Meanwhile, the low-coordination tetrahedral Cu in the CSON is confirmed by X-ray absorption near edge structure (XANES) analyses and Auger electron spectroscopy, which could act as the active site for CO 2 activation and continuous protonation. Ultraviolet-visible-nearinfrared (UV-vis-NIR) spectra and Gibbs free energy diagrams demonstrate that the tetrahedral Cu sites could not only improve the IR light absorption but also reduce the reaction energy barrier for CO 2 reduction. Combining Tauc plot, density functional theory (DFT) calculations, and in situ FTIR spectrum, we propose a photocatalytic mechanism of the activated parity-forbidden electron transfer of d-d orbitals under IR light irradiation (Fig. 1c). As a result, the ultrathin CSON exhibit effective production of CO and CH 4 with the evolution rate of 21.95 and 4.11 μmol g −1 h −1 , respectively, under IR light irradiation, outperforming most reported catalysts under the same reaction conditions (yield < 20 μmol g −1 h −1 ). We further demonstrate the universality of IR light-driven CO 2 reduction based on the d-d electron transfer mechanism by preparing and evaluating various 2D hydrotalcite-like hydroxy salts, like Cu 2 (NO 3 )(OH) 3 nanosheets (CNON), Cu 3 (PO 4 )(OH) 3 nanosheets (CPON) and Cu 2 (CO 3 )(OH) 2 nanosheets (CCON), which provide robust catalyst systems with high performance for photocatalytic CO 2 reduction.

Theoretical design and simulations of 2D hydrotalcite-like hydroxy salts
We first construct the model of 2D CSON with the thickness of a singleunit cell for theoretical simulations (Fig. 2a).
[100] plane is selected as the exposed surface since it is predicted to have the highest surface energy of 1.03 J m −2 compared with [010] facet (0.78 J m −2 ) and [001] facet (0.67 J m −2 ) (Supplementary Fig. 1), suggesting [100] plane has the most excellent activity for catalytic reactions. Moreover, the calculated optical absorption spectra (Fig. 2b) show that there are significant absorption edges in the IR light region (0.6−1.6 eV), which could be caused by the d-d electron transfer. And the intrinsic band gap for photon absorption is around 3.0 eV, indicating that the redox potential of CSON is large enough for simultaneously catalyzing CO 2 reduction and water oxidation.
The DOS and band structure are further calculated to unveil the distribution of orbitals and possible transfer pathways for the excited electrons. As expected, there are apparent splitting d orbitals in the band gap of the spin-down DOS of CSON (Fig. 2c), which is fully consistent with the corresponding band structure shown in Fig. 2d. According to the projected density of state (PDOS) (Fig. 2c) degeneracy of d orbitals in Cu atoms. The hybridization of d orbitals in transition metal atoms and p orbitals in OH ligands leads to the splitting of the metal d electron orbitals, shifting part of the Cu d bands level up to form the empty d bands, which could act as a "cushion step" for d-d electron transition and achieve the cascaded electron transition process. In this respect, valence electrons absorb the IR-light photons and get excited to the middle empty d bands. Subsequently, these activated metastable electrons continue to absorb IR light and transition to the conduction band, enabling them to participate in the CO 2 reduction reaction. Meanwhile, the remaining valence band holes can be utilized for the H 2 O oxidation reaction (Fig. 1c). Therefore, the d-d electron transition mechanism and suitable bandwidth of 2D CSON provide a high potential for IR light absorption and possible activity for IR light-driven CO 2 photoreduction. Similarly, theoretical slab models of CNON, CPON and CCON are also analyzed by DFT calculations. According to the obtained DOS and band structures ( Supplementary  Fig. 2), the splitting empty d orbitals exist in all the Cu-based hydrotalcite-like hydroxy salts (CHHS), indicating the universality of d-d electron transfer mechanism and the huge potential of 2D CHHS for IR light-driven photocatalysis.

Synthesis and characterizations of 2D CSON
Guided by the theoretical simulations, we then synthesized the ultrathin 2D CSON through the solvothermal method (see details in Methods). TEM and HRTEM images in Supplementary Fig. 3a, b show that the obtained CSON have a flake-like morphology with the exposed plane along the [100] direction. The corresponding energy-dispersive spectroscopy (EDS) element mapping images verify the homogeneous distribution of Cu, S and O elements (Supplementary Fig. 3c-f). The thickness of 2D CSON is determined to be ∼1.3 nm by atomic force microscopy (AFM) ( Supplementary Fig. 4), which is consistent with the theoretical thickness of its single-unit cell. Compared with its bulk counterpart, CSON exhibit a high ratio of surface Cu sites with tetrahedral coordination in theory ( Supplementary Fig. 5).
Previous studies have suggested that the tetrahedral Cu atoms generally exhibit higher extinction coefficients in the IR light region, which are expected to be around 50-100 times larger than that of octahedral Cu 13 . In this regard, we also prepared the calcined Cu 4 (SO 4 ) (OH) 6 nanosheets (c-CSON) with abundant surface vacancies by calcining the pristine Cu 4 (SO 4 )(OH) 6 nanosheets (p-CSON) at 300°C in air (see details in Methods), in which the surface -SO 4 vacancies will induce the generation of more tetrahedral Cu atoms. Thermogravimetrydifferential scanning calorimetry (TG-DSC) and thermogravimetrydifferential thermal gravity (TG-DTG) curves were conducted to explore the structural stability of CSON during the calcination process ( Supplementary Fig. 6). The similar TG-DSC and TG-DTG curves of p-CSON and c-CSON confirm that the crystal phase and composition of the two samples are almost identical before and after calcination. Meanwhile, the DSC curves of both p-CSON and c-CSON show that there is no recrystallization or melting process before 300°C, and only a recrystallization peak occurs at 552-560°C, followed by a melting peak at 717-718°C. TEM images in Fig. 3a directly show the clear microporous structure on the surface of c-CSON. Compared with the p-CSON, there is no obvious change in the whole 2D morphology, crystallinity, and element distributions, as displayed in Fig. 3b, c. AFM images in Fig. 3d and Supplementary Fig. 7 confirm that the c-CSON maintain the ultrathin but a porous surface with uneven height profiles. Besides, the X-ray diffraction (XRD) pattern, Fourier-transform infrared spectroscopy (FTIR) spectrum ( Supplementary Fig. 8) and X-ray photoelectron spectroscopy (XPS) analysis (Supplementary Fig. 9) illustrate that the phase and structure of 2D CSON after calcination are well retained. According to the EDS (Supplementary Fig. 10) and XPS composition analyses (Supplementary Table 1), the c-CSON exhibit reduced S content compared with p-CSON, which further corroborates the existence of surface -SO 4 vacancies.
To further demonstrate the tetrahedral-coordination Cu sites, X-ray absorption near-edge structure (XANES) analyses were performed to determine the valence and coordination environment. As depicted in Fig. 3e and the insert, compared with p-CSON, the Cu K-edge position of the c-CSON shifts to a lower energy and the intensity of the white line at round 8996 eV for c-CSON is much weaker. These verify a lower coordination of Cu atoms in c-CSON 18,19 . The To evaluate the catalytic activity of p-CSON and c-CSON for IR light-driven CO 2 photoreduction, photocatalytic experiments were performed under IR light irradiation (simulated by the solar simulator with the AM1.5 and CUT800 filters) without any sacrificial agent at ambient temperature (see more details in the Methods section). The corresponding spectrum of our solar simulator is displayed in Supplementary Fig. 16, in which the IR light component is obtained by using the CUT800 filters. Since the IR light tends to generate heat and raise the temperature of the photocatalyst and reactor 20,21 , a watercooled gas-solid reaction system ( Supplementary Fig. 17a, b) was designed and applied in this work to avoid the effects of thermocatalysis. The average temperature of CSON-based thin film during IR light irradiation has proved to be almost unchanged by the in situ thermographic photographs ( Supplementary Fig. 17c-h). The performance evaluations in Fig. 4g and Supplementary Fig. 18 demonstrate that both p-CSON and c-CSON exhibit excellent activity for CO and CH 4 production under IR light irradiation, in which the calcined sample shows better performance with the CO and CH 4 evolution rate of 21.95 and 4.11 μmol g −1 h −1 respectively, outperforming most reported catalytic systems (yield < 20 μmol g −1 h −1 ) under the similar conditions (Supplementary Table 2). The product selectivity of CO and CH 4 is calculated to 84.2% and 15.8% for c-CSON, 85.7% and 14.3% for p-CSON; while the electron selectivity is 57.2 % and 42.8% for c-CSON, 59.9 % and 40.1 % for p-CSON. Moreover, the apparent quantum yield of c-CSON for IR light-driven CO 2 reduction ( Supplementary Fig. 18b) is calculated up to 0.122% under the monochromatic light of 800 nm wavelength, which is quite high compared with the previously reported IRlight catalysts, such as WO 3 and CuS 3,7 . As for the oxidation end, the remaining holes in the valence band are consumed by water to generate O 2 ( Supplementary Fig. 19). No other liquid or gas products are detected by gas chromatography (GC) (Supplementary Fig. 20) and nuclear magnetic resonance (NMR) spectrum ( Supplementary Fig. 21). In addition, several controlled experiments were conducted to explore the IR light-driven CO 2 reduction reaction over the CSON, including experiments conducted in the dark, without CO 2 , and without catalysts. As shown in Fig. 4g, these experiments confirm that IR light, CO 2 reactant, and catalysts are all necessary for IR light-driven CO 2 reduction. The isotope-labeled 13 CO 2 mass spectrometry for c-CSON ( Fig. 4h and Supplementary Fig. 22a, b) and p-CSON (Supplementary Fig. 22c) was performed to further unveil the source of CO and CH 4 production, in which the photon energy of 14.5 eV is selected for distinguishing the CO and CH 4 according to the absolute photoionization cross sections for CO 2 , CO and CH 4 in Supplementary  Fig. 23. As a result, only 13 CO and 13 CH 4 species are detected after using the isotope-labeled 13 CO 2 as reactants, implying that the products indeed originate from IR light-driven CO 2 reduction for both of them. It is worth noting that c-CSON do not show any significant performance degradation after 96 h long-term photocatalysis (Fig. 4i). The excellent stability of the catalysts is further confirmed by the TEM, HRTEM, XRD patterns, and FTIR spectra as the morphology and crystal structure for c-CSON remain unchanged before and after 96 h continuous photocatalysis (Supplementary Fig. 24).

Mechanistic insights into the CO 2 photoreduction processes
To reveal the electron transition processes by the photoexcitation, the corresponding carrier dynamics characterizations were performed as shown in Supplementary Fig. 25. The transient photocurrent response spectra ( Supplementary Fig. 25a) demonstrate that p-CSON and c-CSON exhibit significant photocurrent density under IR light irradiation, indicating that both samples are capable of achieving IR lightdriven excitation of electron-hole pairs and charge carrier separation. It is notable that c-CSON show higher photocurrent density and smaller transfer resistance ( Supplementary Fig. 25a, b), illustrating the associated higher photoexcited carrier density and rapid electron transition processes. On the other hand, the longer fluorescence lifetime and lower intensity of photoluminescence (PL) spectroscopy indicate that the charge carriers in c-CSON could be easier to separate and participate in the catalytic reactions instead of recombination ( Supplementary Fig. 25c, d). The excellent carrier dynamics observed in c-CSON can mainly be attributed to the presence of more surface Cu sites with tetrahedral coordination and the ultrathin 2D porous configuration. These characteristics not only enhance IR light absorption but also reduce the transfer distance, thereby accelerating charge carrier separation. This, in turn, is beneficial for subsequent IR lightdriven CO 2 reduction.
To further understand the reaction mechanism, in situ FTIR spectroscopy was performed to identify the reaction intermediates during IR light-driven CO 2 reduction in real-time. As displayed in Fig. 5a  h SVUV-PIMS spectrum of the products after 13 CO 2 photoreduction for c-CSON at hν = 14.5 eV. i Cycling measurements for CO 2 photoreduction to CH 4 and CO on c-CSON (using 5 mg sample for this measurement; when a new catalytic cycle begins, the reactor is pumped and refilled with pure CO 2 ). Source data are provided as a Source Data file.
for c-CSON and Supplementary Fig. 26a for p-CSON, a series of new infrared peaks at around 1570 cm −1 were detected for both samples, which are attributed to the COOH* group, a crucial intermediate for CO 2 reduction to CO or CH 4 22,23 . At the same time, the absorption bands near 1116 and 1028 cm −1 belong to the CH 3 O* group, and the peaks at 1064 cm −1 are assigned to the characteristic bands of CHO*; both the CH 3 O* and CHO* groups are pivotal intermediates of CO 2 photoreduction to CH 4 [24][25][26][27] . Moreover, the peaks at 2042 and 2175 cm −1 could be attributed to the adsorbed and free CO products respectively [28][29][30] . It is notable that the intensities of all these peaks for reaction intermediates and products gradually increased with extension of the irradiation time, suggesting the continuous photocatalytic reaction progresses. Besides, the peaks at 1305 and 1208 cm −1 are inferred to the symmetric and asymmetric stretching of HCO 3 * group, while the peaks at 1411 cm −1 are indexed to the CO 3 * group 22,31 . Based on the analysis of in situ FTIR, the most likely reaction pathway for this IR light-driven CO 2 reduction could be proposed as follows: where the asterisks * represent the possible active sites in the catalysts 32 .
Transition metals are noteworthy for their half-filled d orbitals and high charge density, which allow them to not only provide additional orbitals for bonding with reactants but also participate in catalytic reactions by providing electrons 33 . In particular, Cu site has been extensively studied as an active center for CO 2 reduction reactions 34 . In comparing p-CSON with defective c-CSON, we observed a significant increase in low-coordinated Cu (tetrahedral Cu) in c-CSON. As a result, the product yield also increased, but the product selectivity remained largely unchanged. This suggests that c-CSON and p-CSON have similar active centers, with differences only in the amount of catalytic sites. Therefore, tetrahedral Cu is regarded as the active site for CO 2 reduction. According to the detected intermediates of in situ FTIR, the theoretical models of different reaction intermediates were constructed and the corresponding Gibbs free energy was also calculated to determine the reaction pathway and energy barrier of c-CSON (Fig. 5b, c) and p-CSON (Supplementary Fig. 26b, c) during IR light-driven CO 2 reduction. The energy of each elementary reaction is provided in Supplementary Tables 3-5. As the first step, CO 2 adsorption plays an important role in promoting the catalytic performance 35,36 . Given that the basic groups, like -OH and -NH 2 ligands, are usually regarded as the efficient active site for CO 2 adsorption benefiting from the acid-base neutralization effect 37,38 , we first calculated the CO 2 adsorption energy on OH and Cu sites in CSON. As shown in Supplementary Fig. 27, CO 2 molecules prefer to be adsorbed on the surface OH sites than Cu sites due to the lower adsorption energy of the former (−0.02 eV for OH sites and 0.08 eV for Cu sites). More importantly, compared with p-CSON (14.4 m 2 g −1 ), c-CSON with abundant surface vacancies possess about 2-times higher specific surface area (24.2 m 2 g −1 ) according to the Brunauer-Emmet-Teller (BET) plots in Supplementary Fig. 28a, which is beneficial for CO 2 adsorption, as further confirmed by the CO 2 adsorption isotherms in Supplementary Fig. 28b. Subsequently, the adsorbed CO 2 * would move to the nearby Cu sites to be activated and protonated for the formation of COOH* intermediates, which is considered as the rate-determining step (RDS) according to the calculated Gibbs free energy in Fig. 5b.
Moreover, the calculated Bader charge (Supplementary Fig. 29) and charge density distribution (Supplementary Fig. 30) demonstrate that the surface Cu sites in c-CSON have higher electron density than that in the pristine sample, which is helpful for CO 2  Fig. 32). This means that the surface Cu sites in c-CSON are more favorable for adsorbing COOH* intermediates and thus lower the reaction energy barrier. This result fairly agrees with the calculated Gibbs free energy -the energy barrier of the RDS reduces from 1.925 eV for p-CSON to 1.535 eV for c-CSON. As displayed in Supplementary Fig. 33, different surface Cu sites in c-CSON show similar adsorption energy for COOH* intermediates, which suggests that the surface -SO 4 vacancies can significantly reduce the steric hindrance and provide more active Cu sites for CO 2 conversion, therefore promoting the performance of IR light-driven CO 2 reduction.
The generality of d-d electron transfer approach for IR lightdriven CO 2 reduction We demonstrate that the d-d electron transition processes and the underlying reaction mechanisms for IR light-driven CO 2 reduction can also be extended to various hydrotalcite-like hydroxy salts. Therein, we prepared CNON, CPON and CCON samples. TEM and HRTEM images confirm their flake-like 2D thin sheet structures ( Supplementary  Fig. 34a, b, Supplementary Fig. 37a, b, and Supplementary Fig. 40a, b). The corresponding elemental mapping images illustrate the uniform distribution of each element in the counterpart nanosheets (Supplementary Fig. 34c-f, Supplementary Fig. 37c-f, and Supplementary  Fig. 40c-f). According to XRD patterns, their phase is indexed to standard PDF card No. 75-1779 for CNON ( Supplementary Fig. 35b), No. 72-0258 for CPON ( Supplementary Fig. 38b) and No. 72-0075 for CCON ( Supplementary Fig. 41b), respectively. Theoretical slab models show that all these nanosheets possess octahedral (in bulk) and tetrahedral (in surface) Cu coordination structures ( Supplementary Fig. 35a, Supplementary Fig. 38a, and Supplementary Fig. 41a), indicating their potential IR light absorption abilities. As a result, UV-vis-NIR diffuse reflectance spectra ( Supplementary Fig. 35c, Supplementary Fig. 38c, and Supplementary Fig. 41c) clearly demonstrate their similar optical absorption properties with CSON. More impressively, the occurred low-energy absorption edges located around 1.10 eV in the Tauc plots for CNON ( Supplementary Fig. 35c

Discussion
In summary, we have certified a cascaded d-d electron transition mode over the 2D hydrotalcite-like hydroxy salts, which can be used as the general strategy for IR light-driven CO 2 reduction. Various ultrathin Cu-based hydrotalcite-like hydroxy salts are firstly synthesized, for which theoretical and experimental analysis confirm their catalytic performance for IR light-driven CO 2 photoreduction based on d-d electron transition mode. In situ characterizations and DFT calculations unveil the evolutions of active sites, charge carriers, and reaction intermediates. As the result, c-CSON exhibit excellent performance for IR light-driven CO 2 reduction to CO and CH 4 with the rate of 21.95 and 4.11 μmol g −1 h −1 respectively, outperforming most reported IR-responsive catalysts. This work has thus illustrated that the 2D transition metal hydrotalcite-like hydroxy salts have tremendous potential for IR light-driven CO 2 reduction, providing an opportunity for the development of IR-lightresponsive catalysts and paving the way for the design of fullspectrum catalytic systems. Eur., ≥ 25% NH 3 basis) are all acquired from Sigma-Aldrich and were used without any further purification. Deionized (DI) water with resistivity of 18.2 MΩ.cm is obtained by the ultra-pure water system from Stakpure GmbH, Germany.

Catalysts synthesis
Synthesis of p-CSON. A 360 mg portion of CuSO 4 • 5H 2 O was added into the mixture of 8 mL absolute ethanol and 10 mL deionized (DI) water. After vigorous stirring for 30 min, 18 mg hexadecyltrimethylammonium bromide (CTAB) was added into the above solution followed by stirring for another 30 min. Subsequently, 18 mL ammonia solution with a 1% volume concentration and 0.4 mL 1.9 M NaOH solution was slowly dropped into the mixture system. Afterwards, the solution was transferred into a 50 mL Tefon-lined autoclave, sealed and heated at 80°C for 30 min, and allowed to cool to room temperature naturally. The final product was collected by centrifuging the mixture, washed with ethanol and deionized (DI) water for several times until the organic residuals were completely removed, and then dried in vacuum oven at 60°C overnight. The light blue powder was obtained for further usage.
Synthesis of c-CSON. In a typical procedure, the as-obtained ultrathin 2D p-CSON were calcined at 300°C with a heating rate of 10°C min −1 for 1 h under the air atmosphere and then cooled to room temperature. The obtained black green powder was collected for further characterization.
Synthesis of CNON. 2 mmol (483.2 mg) Cu(NO 3 ) 2 • 3H 2 O was added into 30 mL DI water. After vigorous stirring for 10 min, 30 mL NaOH solution with a concentration of 0.1 M was dropwise added into the above mixture followed by stirring for 10 min. Afterwards, the mixture solution was further ultrasonicated for 45 min. The final product was collected by centrifuging the mixture, washed with ethanol and deionized (DI) water for several times until the organic residuals were completely removed, and then dried in vacuum oven at 60°C overnight. The light blue powder was obtained for further usage. Characterization TEM and HRTEM images were performed with a JEOL Jem F-200C TEM with an acceleration voltage of 200 kV. XRD patterns were obtained from a Philips X'Pert Pro Super diffractometer with Cu Kα radiation (λ = 1.54178 Å). X-ray photoelectron spectroscopy (XPS) spectra were acquired on an ESCALAB MKII system with Al Kα (hν = 1486.6 eV) as the excitation source. The binding energies obtained in the XPS spectral analysis were corrected for specimen charging by referencing C 1 s to 284.8 eV. UV-vis-NIR diffuse reflectance spectra were measured on a Perkin Elmer Lambda 950 UV-vis-NIR spectrophotometer. In-situ FTIR spectra were obtained by using a Thermo Scientific Nicolet iS50. Synchrotron-radiation photoemission spectroscopy (SRPES) was executed at the National Synchrotron Radiation Laboratory (NSRL) in Hefei, China. AFM measurements was performed using a Veeco DI Nano-scope MultiMode V system. Ultraviolet photoelectron spectroscopy (UPS) was performed at the Catalysis and Surface Science Endstation at the BL11U beamline of the National Synchrotron Radiation Laboratory (NSRL). The workfunction (WF) was determined by the difference between the photon energy and the binding energy of the secondary cutoff edge. To be exact, E B = hν − (E K + 4.3 -5.0) and WF = hν − (E cutoff − E F ) (E B , binding energy; hν, photon energy; E K , kinetic energy; E cutoff , secondary cutoff edge; E F , Fermi level; photon energy of 40.0 eV and a sample bias of −5 V applied to observe the secondary electron cutoff). CO 2 adsorption isotherms measurements for all the synthetic samples were carried out using an automatic microporous physical and chemical gas adsorption analyser (ASAP 2020 M PLUS). Fluorescence emission decay spectra were recorded with a DeltaFlex-NL (HORIBA Scientific) spectrometer. BET isotherms were conducted by 3 Flex Multiport Surface Characterization Analyzer from Micromeritics.

IR light-driven CO 2 reduction tests
Before performing the CO 2 photoreduction performance, we fabricated the sample into a thin film: the sample was dispersed in deionized water to gain a concentration of about 1 mg mL −1 , and then, through spin-dropping 5 mL of the above dispersion on a quartz glass, followed by heat treatment at 65°C for 30 min, the catalysts thin film could be achieved. During the CO 2 photoreduction process, the catalysts thin film floated on 50 mL of water in a quartz glass vessel with the homothermal condensate water, which could enable the catalysts thin film to retain a constant temperature of 290 ± 0.2 K. A MC-PF-300C Xe lamp with AREF (full spectrum reflectance 200-2500 nm), AM 1.5 G filter and 800 nm cutoff filter was used to simulate infrared light, the corresponding illumination spectrum of which in comparison with sunlight is displayed in Supplementary Fig. 16. Note that the distance from the lamp to the sample was~10 cm, and the irradiation area of sample is around 9.62 cm 2 with an output light density of 71 mW cm −2 .The instrument was initially evacuated for three times, afterwards, pumped by high-purity CO 2 to reach an atmospheric pressure. The gas products were quantified by the Agilent GC-8860 gas chromatograph equipped with TDX-01 column, thermal conductivity detector (TCD) and flame ionization detector (FID) while ultrahighpurity argon was used as a carrier gas (FID detector for carbon-based products and TCD detector for H 2 ). The liquid products were quantified by nuclear magnetic resonance (NMR) (Bruker AVANCE AV III 400) spectroscopy, in which dimethyl sulfoxide (DMSO, Sigma, 99.99%) was used as the internal standard. A recirculating cooling water system was used to keep the photocatalytic system at 290 ± 0.2 K under light irradiation. The product selectivity for CO 2 reduction to CO and CH 4 has been calculated using the following equation: Product selectivity of COð%Þ = ½nðCOÞ=½nðCH 4 Þ + nðCOÞ × 100% ð9Þ Product selectivity of CH 4 ð%Þ = ½nðCH 4 Þ=½nðCH 4 Þ + nðCOÞ × 100% The electron selectivity for CO 2 reduction to CO and CH 4 (2e − for CO, 8e − for the formation of CH 4 ) has been calculated using the following equation: Electron selectivity of COð%Þ = ½2nðCOÞ=½8nðCH 4 Þ + 2nðCOÞ × 100% Electron selectivity of CH 4 ð%Þ = ½8nðCH 4 Þ=½8nðCH 4 Þ + 2nðCOÞ × 100% ð12Þ where n(CH 4 ) and n(CO) are the amounts of produced CH 4 and CO.

Apparent quantum efficiency measurement
The apparent quantum efficiency was defined by the ratio the effective electrons to the total input photon flux of different single wavelength 3 . The number of effective electrons was determined by CO and CH 4 yields of 12 h photoreaction under a monochromatic light wavelength at 800, 850, 940 and 1064 nm. And the total input photon flux of different single wavelength was calculated by the incident light intensity, which was determined using a Silicon-UV enhanced actinometer. The quantum yields for the photocatalytic CO 2 reduction reactions in our paper were determined using the following equation: In situ FTIR spectra experiments All FTIR spectra were recorded on Thermo Scientific Nicolet iS50. The spectra were displayed in transmission units and acquired with a resolution of 4 cm −1 , using 64 scans. The dome of the reaction cell had two KBr windows allowing IR transmission and a third window allowing transmission of irradiation introduced through a liquid light guide that connects to the same IR-light lamp. The catalyst powders were first added to the reaction cell and then trace amounts of water were sprayed on the surface of catalysts. After degassed in N 2 atmosphere for 20 min, the gas flow was switched to high-purity CO 2 until the adsorption is saturated, then the reaction cell was sealed. Next, the FTIR spectra were recorded as a function of time to investigate the dynamics of the reactant adsorption in the dark and desorption/ conversion under irradiation.

Photoelectrochemical measurements
Photocurrent and electrochemical impedance spectroscopy (EIS) were performed using an electrochemical workstation (CHI 660E, CH Instruments, Shanghai, China). Specifically, 10 mg of catalysts were dispersed in a mixture containing 950 µL ethanol and 50 µL Nafion solution, then ultrasonic treated to form homogenous catalyst ink.
Then the catalyst ink was dipped on a polished FTO glass and dried in air. The photocurrent measurements were conducted in a threeelectrode cell system under irradiation of the same light source with that during IR light-driven CO 2 reduction. The FTO glass (2.5 × 1.5 cm 2 ) deposited with materials was used as the photoelectrode, a Pt foil was used as the counter electrode, and Ag/AgCl electrode was used as the reference electrode. The three electrodes were inserted in a quartz cell filled with 0.2 M Na 2 SO 4 electrolyte. The Na 2 SO 4 electrolyte was purged with CO 2 for 1 h prior to the measurements. EIS was measured in the frequency of 1 to 1000 kHz.

DFT calculation details
Density functional theory (DFT) calculations were carried out on a Vienna Ab initio Simulation Package (VASP) 39 . The exchange-correlation potential was described by the generalized gradient approximation (GGA) within the framework of Perdew-Burke-Ernzerhof (PBE) functional 40 . DFT-D3 method was employed to calculate the van der Waals (vdW) interaction 41 . The parameters of dipole correction were applied for the calculation of slab models. Electronic energies were computed with the tolerance of 1 × 10 −4 eV and total force of 0.01 eV/Å. A Monkhorst−Pack k-mesh of 2 × 4 × 1 k-points was used in the structural relaxation, and a kinetic cutoff energy of 450 eV was adopted. The crystal lattice parameters of Cu 4 (SO 4 )(OH) 6 bulk are as follows: a = 13.087 Å, b = 9.865 Å, c = 6.015 Å (α = 90 • , β = 103.33 • , γ = 90 • ) with the P 1 21/a 1 (14) space group. The Cu 4 (SO 4 )(OH) 6 slabs were modeled by the corresponding exposed surface along (100) direction with the thickness of single unit cell. The theoretical approach is based on the GGA with on-site Coulomb interaction parameter (GGA + U method), in which an effective U-J parameter of 5.2 eV was applied to improve the description Cu 3d states 42,43 . A vacuum space of 15 Å was inserted in z direction to avoid interactions between periodic images. All calculations were conducted under spin polarization. The surface energy E s is the energy required to cleave a surface from the corresponding bulk crystal. It can be given by E s = 1=2A ½E s ðunrelaxÞÀN × E b + 1=A ½E s ðrelaxÞ À E s ðunrelaxÞ ð14Þ where A is the area of the surface on the slab models, E s (unrelax) and E s (unrelax) represent the energy of the unrelaxed and relaxed surface slab models, respectively. N is the number of in the slab and E b is the energy of each atom in the bulk counterpart.

Data availability
The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request. Source data are provided with this paper.